1. Field of the Invention
The present patent document relates generally to Computer Aided Design (CAD) software programs, and more specifically to CAD software programs used in the design of integrated circuits and printed circuit boards.
2. Description of Related Art
Computer Aided Design (CAD) software programs are used to create design drawings such as electrical schematics, line drawings, pictorial, and other representations of items which guide the manufacture of those structures depicted in the drawings. Depending upon the particular application, such drawings may be two or three dimensional representations of the subject item.
Such designs usually have to adhere to a set of predefined criteria, referred to as design rules, which are unique to the product, product type, or manufacturing process. Various techniques have been developed to ensure conformance to the design rules. These techniques include the use of design rule checking programs run subsequent to design creation and the use of interactive design rule checking procedures run continually during the design process.
Drawings of photomasks used in the manufacture of integrated circuits and printed circuit boards are composed of design features which define the geometries to be created or processed during specific process steps. For a given photomask, the design features must adhere to minimum design feature width and spacing rules applicable to that process step.
Such CAD systems often use a cursor to create, modify, and delete design features. Among other activities, modification includes stretching, shrinking, moving, and duplicating design features. The cursor is usually manipulated by means of a mouse, a track ball, or by pressing predefined keyboard keys, such as the up, down, left, and right arrow keys. The interactive nature of these systems aids in the timely creation of the design drawings.
Several problems exist with previous CAD software programs. For some design lay-outs, present systems indicate greater than the design rule minimum spacing for adjacent design features. There is also need for design-rule indication between features on two separate photomasks, i.e., inter-level or level-to-level. In addition, during design feature manipulation, it is possible to create features which violate design rules. Correction of such features is time consuming, expensive, and error prone. Thus, there is a significant need to improve the interactive, visual display of design rules and to include automatic conformance to design rules in CAD software systems.